Ink jet printing systems using an intermediate imaging member are well known, such as that described in U.S. Pat. No. 5,614,922. Generally, the printing or imaging member is employed in combination with a print head to generate an image with ink. The ink is typically applied or emitted onto a final receiving surface or print medium by the nozzles of the print head. The image is then transferred and fixed to a final receiving surface. In two stage offset printing, the image is first transferred to the final receiving surface and then transfixed to the surface at a separate station. In other ink jet printing systems, the print head ejects ink directly onto a receiving surface and then the image is fixed to that surface.
More specifically, a solid ink jet or phase-change ink imaging process includes loading a solid ink stick or pellet into a feed channel. The ink stick or pellet is transported down the feed channel to a melt plate where the solid ink is melted. The melted ink drips into a heated reservoir where it is maintained in a liquid state. This highly engineered ink is formulated to meet a number of constraints, including low viscosity at jetting temperatures, specific visco-elastic properties at component-to-media transfer temperatures, and high durability at room temperatures. Once within the print head, the liquid ink flows through manifolds to be ejected from microscopic orifices through use of piezoelectric transducer (PZT) print head technology. The duration and amplitude of the electrical pulse applied to the PZT is very accurately controlled so that a repeatable and precise pressure pulse may be applied to the ink, resulting in the proper volume, velocity and trajectory of the droplet. Several rows of jets, for example, four rows, can be used, each one with a different color. The individual droplets of ink are jetted onto a thin liquid layer, such as silicone oil, for example, on the imaging member. The imaging member and liquid layer are held at a specified temperature such that the ink hardens to a ductile visco-elastic state.
After the ink is deposited onto the imaging member to form the image, a sheet of print medium is removed from a media supply and fed to a preheater in the sheet feed path. After the sheet is heated, it moves into a nip formed between the imaging member and a transfer member, either or both of which can also be heated. A high durometer transfer member is placed against the imaging member in order to develop a high-pressure nip. As the imaging member rotates, the heated print medium is pulled through the nip and pressed against the deposited ink image, thereby transferring the ink to the print medium. The transfer member compresses the print medium and ink together, spreads the ink droplets, and fuses the ink droplets to the print medium. Heat from the preheated print medium heats the ink in the nip, making the ink sufficiently soft and tacky to adhere to the print medium. When the print medium leaves the nip, stripper fingers or other like members, peel it from the imaging member and direct it into a media exit path.
To optimize image resolution, the transferred ink drops should spread out to cover a predetermined area, but not so much that image resolution is compromised or lost. Additionally, the ink drops should not melt during the transfer process. To optimize printed image durability, the ink drops should be pressed into the paper with sufficient pressure to prevent their inadvertent removal by abrasion. Finally, image transfer conditions should be such that nearly all the ink drops are transferred from the imaging member to the print medium. Therefore, efficient transfer of the image from the imaging member to the media is highly desirable.
Efficient transfer of ink or toner from an intermediate imaging member to a media sheet is enhanced by heating a media sheet before it is fed into the nip for transfer of the image. Preconditioning of the recording medium typically prepares the recording medium for receiving ink by driving out excess moisture that can be present in a recording medium, such as paper. Not only does this preconditioning step reduce the amount of time necessary to dry the ink once deposited on the recording medium, but this step also improves image quality by reducing paper cockle and curl, which can result from too much moisture remaining in the recording medium.
Prior art preheaters typically comprised a laminar assembly in which a heating element is adhered to a thermally conductive material, typically Kapton, using a layer of adhesive. Laminating techniques, however, may leave air gaps between the layers making uniform heating difficult. Additionally, insufficient bonding between the layers can cause delamination. Entrapped air and insufficient bonding may lead to stress cracks that can limit the heating element's ability to generate heat homogeneously, which tends to create hot and cold spots along the length of the element.